Modern semiconductor technology is highly dependent on an accurate control of the various processes used during the production of integrated circuits. Accordingly, the wafers have to be inspected repeatedly in order to localize problems as early as possible. Furthermore, a mask or reticle should also be inspected before its actual use during wafer processing in order to make sure that the mask accurately defines the desired pattern. This is done because any defects in the mask pattern will be transferred to the substrate (e.g., wafer) during its use in microlithography. However, the inspection of wafers or masks for defects requires the examination of the whole wafer or mask area. Especially, the inspection of wafers during their fabrication requires the examination of the whole wafer area in such a short time that production throughput is not limited by the inspection process.
Scanning electron microscopes (SEM) have been used to inspect wafers to detect pattern defects. Thereby, the surface of the wafer is scanned using e.g. a single finely focused electron beam. When the electron beam hits the wafer, secondary electrons and/or backscattered electrons, i.e. signal electrons, are generated and measured. A pattern defect at a location on the wafer is detected by comparing an intensity signal of the secondary electrons to, for example, a reference signal corresponding to the same location on the pattern. However, because of the increasing demands for higher resolutions, a long time is required to scan the entire surface of the wafer. Accordingly, it is difficult to use a conventional (single-beam) Scanning Electron Microscope (SEM) for wafer inspection, since this approach does not provide the required throughput.
Wafer and mask defect inspection in semiconductor technology needs high resolution and fast inspection tools, which cover both full wafer/mask application or hot spot inspection. A hot spot inspection refers to an imaging application, wherein only imaging of sensitive areas, which have increased defect potential, is conducted. Presently light optical inspection tools are widely spread for those applications. However, electron beam inspection gains increasing importance because of the limited resolution of light optical tools, which cannot follow the requirements coming along with shrinking defect sizes. In particular, from the 20 nm node and beyond, the high-resolution potential of electron beam based imaging tools become more important to detect all defects of interest. In particular, topographic defects are of special interest and importance.
One drawback of e-beam based optics, e.g. SEM based systems, is the limited probe current within the focused spot. With increasing resolution (decreasing spot size), probe current is further decreasing because of a reduced aperture angle required to control the lens aberrations. Higher brightness sources can provide only limited improvements for the probe current (probe current density) because of the electron-electron interaction. The electron-electron interaction blurs the focused spot and increases the energy width, which again increases the probe diameter, e.g. by introduction of further chromatic aberrations. Consequently, many approaches have been made to reduce e-e interaction in electron beam systems, which are, for example, reduced column length and/or higher column energy combined with late deceleration of the electron beam to the final landing energy just in front of the sample.
Many previous designs gave significant improvements. However, these improvements are still not sufficient to provide a reasonable inspection time for 300 mm diameter wafers. For example, the inspection time should be below 1 hour. Additionally each reduction in defect size by a factor of 2 also requires a spot size reduction by a factor of 2. This results in an increase of pixel numbers by a factor of 4. Such an increase in pixel count reduces the throughput by at least a factor of 4, which is even further reduced by the fact that a smaller electron probe results in a reduced spot current due to a smaller aperture angle, which is required by the aberrations of the lenses involved.
One approach to solve such problems is the use of multiple beams within one column, which reduces the throughput by the number of beams. In order to perform this task using electron microscopic techniques several approaches have been suggested. One approach is based on the miniaturization of SEMs, so that several miniaturized SEMs (in the order of ten to one hundred) are arranged in an array and each miniaturized SEM examines a small portion of the complete sample surface.
Electron beam lithography faces similar throughput limitation to get reasonable writing times. Electron beam lithography also considers multi-beam approaches. However, e-beam lithography applications cannot be implemented for e-beam inspection in light of different boundary conditions, which avoid problems occurring for e-beam inspection. For electron beam lithography, the beam energy is higher as compared to inspection (lithography 50 keV-100 keV, inspection typically 100 eV-5 keV). The higher beam energy reduces the impact of e-e interaction. Additionally, no detection is needed from the surface images produced by the individual beams. For example, multi-beam electron projection/lithography systems are used to create patterns of variable shape on a substrate by switching on and off individual beams. However, beyond the different boundary conditions mentioned above, there are also differences in the devices. For example, an aperture or aperture array is focused on the sample, on which the pattern is generated.
Accordingly, there is a need for an imaging charged particle beam device which provides a sufficient resolution and which is able to increase the data collection to such an extent that the device can also be applied to high speed wafer inspection.